For wireless communication apparatuses using a multi-phase shift keying modulation system for a high efficiency transmission, significant design conditions for reducing power leakage to adjacent channels by reducing nonlinear distortion of a power amplifier in a transmission unit (hereinafter, described as a “transmission amplifier” in some cases) are provided. For example, in a case in which a channel uses a frequency at a boundary of a frequency band assigned to a carrier, noise occurs to adjacent channels for other carriers because of power leakage from the channel in a wireless communication.
Distortion compensation amplifiers that compensate for nonlinear distortion employ a pre-distortion technique in some cases. In the pre-distortion technique, a transmission signal is corrected in advance using data having characteristics inverse to characteristics of nonlinear distortion (hereinafter, described as “distortion compensation data” in some cases). The corrected signal is used as a signal to be input to a transmission amplifier. Furthermore, in the pre-distortion technique, in order to update the distortion compensation data for changes that occurred in the transmission amplifier over time, one portion of the transmission signal amplified by the transmission amplifier is fed back. The transmission signal is compared with the transmission signal prior to being corrected using the distortion compensation data, and the distortion compensation data are updated. Hereinafter, a loop system including the transmission amplifier and a feedback system that are used to update the distortion compensation data is described as a “distortion compensation loop” in some cases.
Accordingly, a signal that is fed back in the distortion compensation loop (hereinafter, described as a “feedback signal” in some cases) is required that the signal has characteristics of an output signal of the transmission amplifier.
However, the transmission signal is reflected by an antenna because of fluctuation in load imposed on the antenna that is caused by deterioration of conditions for placing the antenna or the like. Then, in some cases, the reflected transmission signal (hereinafter, described as a “return signal” in some cases) leaks into the distortion compensation loop. Accordingly, the feedback signal is not considered as a signal that is obtained in consideration of the output signal of the transmission amplifier. As a technique for solving the above-mentioned problem, a technique is disclosed, in which a vector adjustment circuit capable of performing phase adjustment or amplitude adjustment is provided, and in which cancellation of a return signal that has leaked into a feedback signal is performed (see International Publication Pamphlet No. WO2006/059372).